Month: March 2012

Over the last few months, on different systems I have been modifying the ib_log_files and their sizes, increasing them due to the increase in load they have been experiencing. These files are used to contain information which has been committed, but not yet written to the ibdata files or the individual .ibd files if using innodb_file_per_table. And these files are used on recovery if the server shuts down unexpectedly during recovery processing to get the system back into a consistent state again.

The checkpointing in MySQL is well documented, but I was surprised just how much difference the throughput can change for heavy write applications if the sizes of these files is changed. It may seem obvious but given how disruptive it can be to shut down a system, change these values and start it up again, this process is not done frequently. These files help improve performance as changes are written linearly into the file, compared to the changes in the data files which are written in pages and those pages are probably scattered randomly over the disk, the latter process being slower.

I come from a Sybase background where “a database” consists of “data” and “log” files. The data files correspond with the ibdataX or .ibd files and the log files correspond with the “ib_log_fileX” files, the major difference being that (at least when I looked at this in Sybase 12.5) the log files were considered part of the database and once added you also could not reduce their size. Increasing was easy but then you couldn’t free up the space again.

So MySQL and Sybase are similar, but MySQL’s implementation keeps the ib_logfileX files independent from the database files. It struck me that it would be very useful if we could dynamically tailor the two settings that InnoDB provides: the innodb_log_file_size and the innodb_log_files_in_group. Since MySQL already automatically creates missing files, typically done if you start MySQL for the first time, or if you remove the files after changing the innodb_log_file_size parameter, this sounds reasonably easy. Make the innodb_log_files_in_group larger and then when you get past the last available file, you build a new one. If the number is decreased then you simply go back to the first one again, ignoring later files. Changing the size of the files could be done too, but perhaps is harder. In both cases you need to be sure this works when recovering from a crash, which is a critical moment.

A change like this would help people see whether changing the size improves performance, and certainly for systems where taking downtime is expensive and difficult, this would be a welcome move, as over time initial sizing guidelines may vary and a dynamic change would resolve the problem quickly. I recently took down some old MySQL systems which had been running uninterrupted for just over a year. I’m sure I would have probably adjusted these settings earlier had I been able to, and that would have been very convenient.